The following Patent Literature 1 describes a hydraulic brake system including a master cylinder configured to supply a pressurized working fluid to a brake device, a regulator configured to supply a pressure-regulated working fluid to the master cylinder, and a working-fluid supply device configured to supply, to the regulator, a working fluid having a pressure adjusted to a given level by control. The master cylinder includes (A-1) a housing including a partition portion which partitions an interior of the housing into two fluid chambers and which has an opening, (A-2) an input piston disposed in the housing so as to be located rearward of the partition portion and connected to a brake operation member so as to be moved forward by an operation of the brake operation member, (A-3) a pressurizing piston having a main body portion which is disposed in the housing so as to be located forward of the partition portion and which has a flange formed at a rear end thereof, (A-4) an inter-piston chamber defined between the input piston and the pressurizing piston utilizing the opening of the partition portion, (A-5) a pressurizing chamber which is defined forward of the pressurizing piston and in which the working fluid to be supplied to the brake device is pressurized by a forward movement of the pressurizing piston, (A-6) an input chamber which is defined between the flange of the pressurizing piston and the partition portion and to which is introduced the working fluid for giving the pressurizing piston a force that moves the pressurizing piston forward, and (A-7) an opposing chamber defined forward of the flange of the pressurizing piston so as to be opposed to the input chamber with the flange interposed therebetween.
The master cylinder is configured such that its pressurizing state is switched selectively between (I) a first pressurizing state in which the pressurizing piston is moved forward depending on both of a pressure of the working fluid in the inter-piston chamber and a pressure of the working fluid in the input chamber by shutting off communication between the inter-piston chamber and the opposing chamber with a communication switching valve placed in the closed state and by allowing communication between the opposing chamber and a low-pressure source with a low-pressure-source shut-off valve placed in the open state and (II) a second pressurizing state in which the pressurizing piston is moved forward depending on only the pressure of the working fluid in the input chamber by allowing the communication between the inter-piston chamber and the opposing chamber with the communication switching valve placed in the open state and by shutting off communication between: the inter-piston chamber and the opposing chamber; and the low-pressure source with the low-pressure-source shut-off valve placed in the closed state.
The regulator is configured to supply, to the input chamber of the master cylinder, the working fluid regulated in one of (I) a first pressure-regulating state in which a pilot piston comes into contact with a valve member and moves with the valve member by a first pilot pressure which is a pressure of the working fluid supplied from the pressurizing chamber of the master cylinder, so that the regulated pressure is regulated to a level in accordance with the first pilot pressure and (II) a second pressure-regulating state in which the valve member moves in a state in which the pilot piston is separated away from the valve member by a second pilot pressure which is a pressure of the working fluid supplied from a pressure adjuster, so that the regulated pressure is regulated to a level in accordance with the second pilot pressure.